1. Field of the Invention
The present invention relates to a rewritable thermal label of the non-contact type and, more particularly, to a rewritable thermal label of the non-contact type which allows recording and erasure of information repeatedly in accordance with the non-contact method while the rewritable thermal label remains attached to an adherend, allows using a substrate having a poor solvent resistance and can be recycled together with the adherend.
2. Description of Related Art
Currently, labels for control of articles such as labels attached to plastic containers used for transporting foods, labels used for control of electronic parts and labels attached to cardboard boxes for control of distribution of articles are mainly labels having a heat-sensitive recording material such as direct thermal paper as the face substrate. In the heat-sensitive recording material, a heat-sensitive recording layer containing an electron-donating dye precursor which is, in general, colorless or colored slightly and an electron-accepting color developing agent as the main components is formed on a support. When the heat-sensitive recording material is heated by a heated head or a heated pen, the dye precursor and the color developing agent react instantaneously with each other and a recording image is obtained. When an image is formed on the heat-sensitive recording material, in general, it is impossible that the formed image is erased so that the condition is returned to that before the image is formed.
In the label for control of articles described above, the face substrate is formed mainly by using the above heat-sensitive recording material. Informations such as the addresses to be sent, the name of the sender, the number and the lot number and a bar code expressing the informations are printed on the label using a thermal printer of the contact type and the label having the printed information is attached to an adherend. When the label completes the expected role, the label is manually removed from the adherend such as a container and a card board box to reuse the adherend and great amounts of labor and time are required for the removal of the label. To the adherend from which the label has been removed, another label printed by using a thermal printer of the contact type is attached and the adherend is reused repeatedly in this manner.
It is the actual situation that a label is attached and removed every time an adherend is used. A rewritable thermal label which allows repeated recording and erasure of information while the label remains attached to the adherend, without removing the label every time the adherend is used, has been desired.
On the other hand, in recent years, reversible heat-sensitive recording materials which allow recording and erasure of an image, such as (1) a reversible heat-sensitive recording material having a heat-sensitive layer which is formed on a substrate and contains a resin and an organic low molecular weight substance showing reversible changes in transparency depending on the temperature and (2) a reversible heat-sensitive recording material having a heat-sensitive color development layer which is formed on a substrate and contains a dye precursor and a reversible color developing agent, have been developed.
When the above reversible heat-sensitive recording material is applied to the above rewritable thermal label, it is required that information be recorded and erased in accordance with the non-contact method since the information is recorded and erased while the label remains attached to an adherend. Therefore, the reversible heat-sensitive recording material described above in (2) is preferable.
However, in the reversible heat-sensitive recording material described above in (2), a coating fluid prepared by dissolving or dispersing a dye precursor, a color developing agent and other additives used where necessary in a solvent such as tetrahydrofuran is used for forming the heat-sensitive color development layer. Therefore, films of resins which are mainly used for the substrate such as polystyrene, acrylonitrile-butadiene-styrene copolymers (ABS resins) and polycarbonates cannot be used due to the poor resistance to solvents and the resin used for the substrate is limited to resins having the excellent resistance to solvents such as polyethylene terephthalate and polypropylene. Thus, the above reversible heat-sensitive recording material has a drawback in that the type of the resin used for the substrate is limited. To use the above resins mainly used for the substrate of the label as the substrate of the above label, it is necessary that the resistance to solvents be improved.
In general, laser beam is used for recording information in accordance with the non-contact method using the reversible heat-sensitive recording material described above in (2). Therefor, it is important that the material has the function of absorbing laser beam and efficiently converting the absorbed laser beam into heat.
Moreover, it is required that the adherend such as a plastic container be recycled after the use so that the society of the resources-recycling type can be constructed. When the plastic container is recycled, it is desirable that the rewritable thermal label can be recycled together with the adherend while the label remains attached to the adherend.